1. Field of the Invention
The present invention is related to gas sensing using tunable diode laser absorption spectroscopy, and more particularly, is related to an apparatus and method for nondestructive monitoring of gases in sealed optically transparent containers, using tunable diode laser absorption spectroscopy, while reducing or eliminating etalon interference fringes.
2. Background and Material Information
Tunable diode laser absorption spectroscopy (TDLAS) is a highly selective and versatile technique for measuring many trace atmospheric constituents with detection sensitivities in the sub-parts-per-billion (ppbv) concentration range. Other spectroscopic techniques such as Fourier-transform infrared (FTIR) spectroscopy, which record the interaction of infrared radiation (IR) with experimental samples, measuring the frequencies at which the sample absorbs the radiation and the intensities of the absorptions, do not have as high a sensitivity as TDLAS in the near infrared (NIR) region of the spectrum. Specifically, FTIR spectroscopy can only achieve minimum detectable absorbencies of 10xe2x88x923-10xe2x88x924 in a 1 Hz detection bandwidth, whereas TDLAS can achieve 10xe2x88x926-10xe2x88x927 in the same detection bandwidth. Thus it is desirable to use TDLAS over FTIR in the NIR spectral region where many atmospherically important constituents have weak molecular overtone and combination absorption bands.
TDLAS utilizes the wavelength response of matter to probe physical and chemical properties. Also, TDLAS determines the concentration of a gas by measuring the amount of light absorbed at a particular wavelength. The intensity of light absorbed is directly related to gas concentration through Beer""s law:
ln(Ix/Io)=xe2x88x92n"sgr"x
where Ix is the intensity of the transmitted light, Io is the intensity of incident light, n is the concentration gas, "sgr" is the absorption cross section, and x is the pathlength.
Applications for laser absorption spectroscopy range from basic chemical kinetics research and environmental monitoring to medical diagnostics and industrial process monitoring. When trace gas concentrations need to be measured, a system must be designed to minimize noise and background signals.
The amplitude of tunable diode laser absorption signals is proportional to the distance, x, (or pathlength) over which a target gas is sampled. Multipass configurations can increase the pathlength, but require the use of collimated laser beams to ensure a laser beam of sufficient power reaches the detection circuitry. In situations where the target gas resides in a sealed optically transparent container, the laser beam must pass through the container walls to sample the target gas. U.S. Pat. No. 5,317,156 which issued to Cooper et al. on May 31, 1994, shows a typical TDLAS system using a multi-pass sample cell.
Most TDLAS systems are limited in sensitivity not by laser or detector noise but by optical fringes superimposed on the measured spectrum. These result from unwanted optical artifacts, or etalons, formed by reflections and scattering in the optical system. FIG. 1 shows a collimated laser beam 11 having a width defined by edges A and B passing through an optically transparent material, such as a glass container wall W. Partial reflected rays, shown as Bxe2x80x2, occur at the air/container interface. The partial rays Bxe2x80x2 from one ray are reflected within the wall W and overlap with adjacent incident rays, shown as ABxe2x80x2 in FIG. 1, causing interference. These overlapping rays ABxe2x80x2 are phase shifted with respect to each other due to the different optical pathlengths traversed and cause constructive interference when the phase shift is 0, and destructive interference when the phase shift is xcfx80. The interference effect from these overlapping laser beams creates the unwanted etalons.
When the interfering beams are incident on a detector (e.g. a photodetector or square law detector) and the laser frequency is scanned, the phase relationship varies, producing a periodic intensity variation in the photo current. The interference pattern in most cases limits the sensitivity of the measurement and obscures small absorption signals. FIG. 2 shows a molecular absorption graph of a TDLAS signal S using a collimator lens. The signal is recovered in a single pass through a sample container and with no signal averaging. The distorting effects of the etalons are evident; the amplitude and frequency of gas absorption feature G is comparable to the amplitude and frequency outside OFQ the gas absorption feature G as a result of interference fringes F, which may obscure the reading, especially when analyzing small concentrations of gas.
The art is replete with devices and techniques that attempt to reduce etalons. These techniques can be categorized as follows; (I) mechanical modulation or dithering of the etalon spacing (ii) modified modulation schemes (iii) background subtraction and (iv) post-detection signal processing.
If the etalon pathlength-difference is mechanically modulated then the fringes will shift relative to the absorption spectrum. As the spectrum is averaged, the fringes will average to zero, provided the modulation amplitude corresponds either to an integral or a large number of fringes. One method of accomplishing this modulation has been demonstrated by Webster, in U.S. Pat. No. 4,684,258, which issued on Aug. 4, 1987. This patent discloses the interposing of an oscillating Brewster plate into the beam at a point between the two surfaces forming the etalon. Oscillating the plate (by typically 1xc2x0) varies the optical pathlength through the plate. Because the plate is at the Brewster angle, reflection losses are minimized. One disadvantage of this method that it is difficult to apply to a multi-pass cell without causing significant attenuation of the beam.
U.S. Pat. No. 4,934,816 which issued to Silver et al. On Jun. 19, 1990, discloses a method using a piezoelectric transducer (PZT) to vibrate the mirror or other component which forms one surface of the etalon. However, both the Webster and Silver techniques are difficult to implement when the sample containing the gas to be analyzed is a sealed product container that must be sampled in-situ.
In TDLAS systems using wavelength modulation spectroscopy (WMS), the fringes can be averaged to zero by applying an additional low-frequency wavelength modulation to the diode laser, with an amplitude equal to an integral number of periods of the etalon fringe. As a result, the technique is effective only at removing fringes with a period less than the absorption line width since the modulation amplitude needed to remove longer period fringes would also smear the absorption line shape and reduce its peak height.
For a perfectly stable system a background spectrum, obtained by supplying zero air to the instrument inlet, would display the same etalon fringes as the sample spectrum. Subtraction of this background spectrum would then remove the fringes. Real systems however are subject to thermal drift, so that in the time between taking the sample spectrum and the background spectrum the fringes will have drifted and cancellation will not be perfect. Thus, the success of background subtraction depends firstly on the thermal and mechanical stability of the system and secondly on the rapidity with which sample and background spectra can be alternated.
Post-detection signal processing can take the form either of analog processing of the signal from the look-in amplifier or demodulator, or digital processing of the signal acquired by the signal averager. Both take advantage of the periodic nature of the optical fringes. A simple low-pass filter following the lock-in amplifier can dramatically reduce fine-pitch fringes. In known TDLAS systems, a combination of background subtraction with some form of post-detection processing is most commonly used.
The minimum detectable absorption is given by the smallest variation in ln(Ix/Io) that can be distinguished from noise or background signals. Sources of noise include laser intensity fluctuations and detector thermal noise. Many TDLAS techniques exist that overcome these noise sources, such as dual beam cancellation, wavelength modulation and single tone and two tone frequency modulation. In principle, these techniques are capable of achieving shot noise limited detection, which is the fundamental sensitivity limit determined by the statistical nature of converting a photon into an electron at a detector.
In most practical systems using coherent light sources, however, background signals due to optical interference limit detection sensitivities. Optical interference is observed whenever two or more light waves which have traveled different optical pathlengths simultaneously arrive at a detector. For example, in laser absorption spectroscopy, a collimated beam, after passing a transmissive optical element, may be broken up into a number of partial waves which travel along different optical pathlengths before arriving at a detector. As the frequency of the laser is scanned, the relative phases of these partial waves vary, resulting in constructive and destructive interference. The physical manifestation of such interference is a periodic variation in the detected intensity.
When interference-related intensity variations are comparable in amplitude to intensity variations due to gaseous absorption, then measurements of gas concentration become imprecise. Many methods have been developed to reduce the amplitude of interference fringes. However, each method has limitations and such methods are not generally applicable to all situations encountered in laser absorption spectroscopy. Furthermore, the previously-mentioned techniques for reducing etalons are often unreliable and are overly complex, as they use opto-mechanical or electrical techniques which, as described above, are not generally suitable for all situations, particularly when measurements need to be made in-site through sealed product containers. Further, none of the previously-mentioned techniques will reduce interference effects which arise when analyzing the contents of a sealed container.
Many new and existing commercial products have ingredients that are sensitive to atmospheric gases. Prolonged exposure to elevated levels of these gases degrades product quality and potency. Throughout processing, particularly aseptic processing (where sterilization is through filtration), products are held in and transferred between containers where the potential exists for gas exposure. When processing gas-sensitive formulations, ambient levels of reactive gases are reduced with inert gas purges. However, trace quantities of reactive gases can be entrained into solution containers and trapped in the container headspace (i.e., the region above the product and below the seal).
Many known apparatus and methods of gas analysis require the destruction of the container in order to analyze the gas contained therein. Production monitoring of headspace gas is often performed off-line at scheduled intervals. Because this method is slow, the potential for large quantities of off-specification product being produced is significant. One such destructive method is discussed in U.S. Pat. No. 5,900,378, which issued to Mayer et al. on May 4, 1999. Analysis requiring the destruction of the container is not only time-consuming, but is costly for the manufacturer, and is both labor intensive and requires disposal of what may otherwise be perfectly good product. Thus, a fast, sensitive, and nondestructive method and apparatus for monitoring gas concentrations in both on-line and off-line applications are desired.
The present invention provides a system and method for the nondestructive detection of gas or gas composition in a sealed container. The system of the present invention includes a tunable diode laser source that provides a diverging (i.e., uncollimated) laser beam for absorption in a substance to be analyzed or measured, a detector that detects the laser beam, a zone that accepts one or more of the sealed containers, each container being substantially optically transparent and containing the substance to be measured, the laser source configured to transmit the laser beam through the zone, and a collection lens that focuses the laser beam onto the detector.
The detector of the system may be a square law detector. Also, the collection lens may be convex or biconvex.
The system may further have a diverging lens that diverges the laser beam and that is located between the laser source and the zone. Additionally, the system may include an optical fiber coupled to the source that transmits light to the zone. Also, the collection lens may be stationary.
The invention may further include a reflector, the zone being positioned between the source and the reflector, the laser source configured to transmit the laser beam through the zone and onto the reflector, the reflector returning the laser beam to the collection lens, with the reflector positioned on an opposite side of the zone from the detector, the collection lens and the source.
The method of the present invention includes placing the sealed container in an acceptance zone, providing a diverging laser beam, transmitting the uncollimated laser beam through one wall of the sealed container, transmitting the uncollimated laser beam through another wall of the sealed container, the another wall being opposed to the one wall, focusing the laser beam onto a surface of a detector, and detecting, using a detector, the focused laser beam.
The focusing of the laser beam may include using a collection lens, which may be either convex or biconvex.
The method may further include diverging the uncollimated laser beam before transmitting the uncollimated laser beam through the one wall of the container. The diverging may include using a diverging lens, and the diverging lens may be located between the laser source and the zone.
Additionally, the method may further include transmitting the laser beam through an optical fiber coupled to the source, and transmitting, from the optical fiber, the uncollimated laser beam to the zone. Also, the collection lens may be stationary.
The method may further include collecting the uncollimated laser beam after transmitting the uncollimated laser beam through another wall of the sealed container, reflecting the uncollimated laser beam back through the another wall of the sealed container, and reflecting the uncollimated laser beam back through the one wall of the sealed container.
The present invention is particularly useful for detection of oxygen, moisture and carbon dioxide, and other gases in small volume product containers during pharmaceutical, food and beverage, and consumer electronic production. The present invention can analyze gas samples at online production speeds, can measure trace quantities and can nondestructively analyze gas contents within sealed containers.
When testing small container volumes, a laser beam need not be collimated to ensure sufficient laser power reaches the detector. By allowing the laser beam to diverge at its natural rate, fewer partial rays are incident onto the container at normal incidence. Therefore fewer overlapping rays, having traveled along different optical pathlengths, will be incident on the detector and etalon interference will be minimized. This reduction in background interference improves the signal to background noise ratio and permits trace level gas detection in small diameter containers at relatively high speeds, usually two seconds or less. The diverging beam may be collected using a collection lens such as a biconvex lens, after traversing the container, and focused onto a detector.